Our goal is a thorough understanding of excitation and excitation-contraction coupling in the heart. This includes both normal mechanisms and those involved in arrhythmogenesis. The goal for the period of this request is to analyze plateau currents of ventricular muscle in single cardiac cells. Cells will be isolated from adult mammalian ventricle, and voltage-clamped by the patch electrode whole cell voltage clamp. Intracellular dialysis, to control the intracellular ionic environment, will be an important tool in separating membrane currents for analysis. The project will begin with efforts to establish the number and behavior of channel-mediated calcium currents in the ventricle, using intracellular dialysis to block potassium currents and to prevent calcium-activated currents. Simultaneously, we will try a promising approach to define the steady-state current-voltage relation of sodium-calcium exchange current in the ventricle. This effort will also use intracellular dialysis, to achieve the control of intracellular solutions necessary if the properties of electrogenic sodium-calcium exchange are to be understood. The specific aims are: 1) determine the number of calcium currents and their properties in isolated, dialyzed ventricular cells; 2) determine the nature and kinetics of the delayed rectifier, IK, in isolated dialyzed ventricular cells; 3) determine the nature and kinetics of the early outward current, Ieo, in ventricular myocytes; 4) determine the steady-state current-voltage relation of Na-Ca exchange current, INaCa; and 5) test the hypothesis that a significant component of slow inward current in cardiac cells is produced by a transient change of INaCa. This is an ambitious proposal, which is intended to answer key questions about electrical activity and its relation to contraction in heart.